The present invention relates to a method for homogeneously mixing a uranium/plutonium mixed oxide powder, and more particularly to a method wherein a plurality of powders having different densities (specific gravities), such as a uranium oxide powder, a plutonium oxide powder, and a dry recovered powder, are homogeneously mixed together by means of a jet mill and are withdrawn in a homogeneously mixed state. This technique is useful for the preparation of uranium/plutonium mixed oxide pellets.
The preparation of a uranium/plutonium mixed oxide (MOX) fuel involves the steps of weighing predetermined amounts of a uranium oxide (UO.sub.2) powder, a plutonium oxide (PuO.sub.2) powder, a dry recovered powder prepared by grinding a uranium/plutonium mixed oxide sinter (a powder prepared by grinding sintered uranium/plutonium mixed oxide pellets having, for example, defective appearances, i.e., scrap pellets), and the like so as to give a predetermined plutonium enrichment (percentage addition) and homogeneously mixing the powders. A ball mill or an attritor mill has hitherto been used in the step of homogeneously mixing the uranium/plutonium mixed oxide powder.
In the ball mill, the feed and recovery of the powders are carried out in such a manner that the powders in a vessel are poured as such into the mill and, after the mixing, the homogeneous powder mixture in the mill is recovered in the vessel by tilting the mill. This method is advantageous in that the powders can be satisfactorily homogeneously mixed together and that the operating conditions (number of revolutions and time) can be readily set, therefore it has been extensively used in the art. The method, however, suffers from low mixing efficiency and the necessity for mixing for a long period of time. Further, the size of the apparatus has been increased for the throughput. In particular, in the preparation of a plutonium-containing nuclear fuel material, the apparatus should be installed within a glove box. Therefore, the apparatus is restricted by the size of the glove box and the consideration of maintenance, therefore a large-sized ball mill cannot be installed. An additional problem involved in the ball mill is that the treatment should be carried out batch-wise.
On the contrary, in the case of the attritor mill, the feed and recovery of the powders are carried out in such a manner that the powders are fed into the mill while vibrating the powders by means of a vibration feeder and that the resultant homogeneous powder mixture is discharged from the mill through piping. In this case, the treatment can be carried out continuously, is suitable for treatment of large amounts of powders, and can be carried out with high mixing efficiency. As with the ball mill, however, the attritor mill has a rotating section driven by a motor and hence is poor in maintenability. Further, since heat of friction is generated during mixing, a cooling mechanism should be provided in order to prevent oxidation of the uranium/plutonium mixed oxide powder. This poses an additional problem that the size of the apparatus should be further increased. Furthermore, due to the construction of the mill, the powders are likely to stay within the mill, leading to an increased exposure dose.
As is well known in the art, jet milling is a method for pulverizing a powder. A jet mill is an apparatus wherein particles are accelerated with the aid of a high-speed gas stream to allow the particles to collide with one another to conduct pulverization. The jet mill is advantageous in that continuous treatment and mass treatment are possible, that the generated heat of friction can be immediately removed, that the size of the device can be reduced, and that the maintenability is good. For these reasons, the jet mill has been used, for example, for pulverizing uranium/plutonium mixed oxide pellets having defective appearances (scrap pellets) to prepare a dry recovered powder. The jet mill, however, has not been used for homogeneously mixing a uranium/plutonium mixed oxide powder.
The jet mill is equipment which has been originally intended to be used for the pulverization of a ceramic powder but has not been extensively used for mixing purposes. This is because, although the jet mill has the function of mixing the powder, a possible range of mixing is limited. The reason for this is as follows. In the jet mill, the powders are fed and discharged in a manner utterly different from that in the case of the abovedescribed ball mill and the like. Specifically, the powders are fed into the jet mill with the aid of compressed air and, after the pulverization, the mixture of the pulverized powder with the air stream is discharged from the jet mill and then separated into the powder (solid) and the gas by means of a cyclone, a bag filter or the like, followed by the recovery of the separated powder only in a vessel.
In the above-described method for feeding and discharging powders using the jet mill, when powders having different compositions with different densities are mixed, unfavorably the dissimilar powders thus mixed are separated again from one another due to their difference in density in the course of the separation of the solid from the gas after the discharge. This leads to a variation in the composition of the resultant powder mixture. Therefore, the contemplated homogeneous mixing cannot be achieved. For this reason, the jet mill has been used in most cases in pulverization of ceramic powders having one and the same composition (for example, a powder prepared by crushing a sinter forming a solid solution on an atomic level) and the like, but has not been used in applications where dissimilar powders having different densities are homogeneously mixed together (mixing while pulverizing).
As described above, in the preparation of a uranium/plutonium mixed oxide fuel, a uranium oxide powder, a plutonium oxide powder, a dry recovered powder and the like should be homogeneously mixed together while pulverizing. If the homogenization-mixing is unsatisfactory, a portion rich in the plutonium component, called a "plutonium spot", is created within the sintered pellet. The presence of such a plutonium spot causes this portion to intensively undergo fission during exposure of the pellet, creating a high-temperature hot spots The plutonium spot present within the pellet has no significant influence. On the other hand, when the plutonium spot is present around the surface of the pellet, that portion becomes hot, greatly affecting a metallic cladding tube. In particular, the rise of the spot temperature sometimes causes the cladding tube to be melted, leading to a serious trouble, that is, fuel failure. For this reason, the size of the plutonium spot and the plutonium concentration of the pellet are strictly restricted, so that preferably the plutonium concentration should be as uniform as possible and the diameter of the plutonium spot should be as small as possible.
Various powders used in the preparation of a uranium/plutonium mixed oxide fuel are significantly different from one another in powder density. Specifically, the density of the plutonium oxide powder, the lowest-density powder, is about 2 g/cc, whereas the density of the dry recovered powder, the highest-density powder, is about 6 g/cc, that is, three times larger than that of the lowest-density powder. For this reason, when the jet mill is used, although the homogenization-mixing per se in the jet mill can be successfully carried out without posing any problem, the powders are again separated from one another due to their density difference in the course of the separation of the powder mixture from the gas after the discharge from the mill. Therefore, a plutonium-rich portion and a plutonium-lean portion are created in the powder, so that the contemplated satisfactory homogenization cannot be achieved.